U.S. patent application number 09/859844 was filed with the patent office on 2001-11-22 for method of working piezoelectric substance and method of manufacturing composite piezoelectric substance.
This patent application is currently assigned to OLYMPUS OPTICAL CO., LTD. Invention is credited to Abe, Takashi, Esashi, Masayoshi, Wakabayashi, Katsuhiro.
Application Number | 20010042291 09/859844 |
Document ID | / |
Family ID | 18652955 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010042291 |
Kind Code |
A1 |
Esashi, Masayoshi ; et
al. |
November 22, 2001 |
Method of working piezoelectric substance and method of
manufacturing composite piezoelectric substance
Abstract
A method of working a piezoelectric substance, which comprises
the steps of, forming, on one surface of a piezoelectric block, an
etching mask having an aperture which defines a boundary region
between a first piezoelectric segment to be removed, and a second
piezoelectric segment to be left remained, forming, on the opposite
surface of the piezoelectric block, a sacrificial layer which
corresponds to the first piezoelectric segment to be removed and
the boundary region, etching the piezoelectric block in the
boundary region to reach the sacrificial layer, and eliminating the
sacrificial layer to remove the first piezoelectric segment.
Inventors: |
Esashi, Masayoshi;
(Sendai-shi, JP) ; Abe, Takashi; (Sendai-shi,
JP) ; Wakabayashi, Katsuhiro; (Hachioji-shi,
JP) |
Correspondence
Address: |
Scully, Scott, Murphy & Presser
400 Garden City Plaza
Garden City
NY
11530-0299
US
|
Assignee: |
OLYMPUS OPTICAL CO., LTD
TOKYO
JP
|
Family ID: |
18652955 |
Appl. No.: |
09/859844 |
Filed: |
May 17, 2001 |
Current U.S.
Class: |
29/25.35 ;
29/846; 29/847; 29/852 |
Current CPC
Class: |
H01L 41/37 20130101;
Y10T 29/42 20150115; H01L 41/335 20130101; Y10T 29/49155 20150115;
Y10T 29/49156 20150115; Y10T 29/49165 20150115; H01L 41/332
20130101 |
Class at
Publication: |
29/25.35 ;
29/846; 29/847; 29/852 |
International
Class: |
H04R 017/00; H05K
003/02 |
Foreign Application Data
Date |
Code |
Application Number |
May 18, 2000 |
JP |
2000-146630 |
Claims
What is claimed is:
1. A method of working a piezoelectric substance, which comprises
the steps of; forming, on one surface of a piezoelectric block, an
etching mask having an aperture which defines a boundary region
between a first piezoelectric segment to be removed, and a second
piezoelectric segment to be left remained, forming, on the opposite
surface of said piezoelectric block, a sacrificial layer which
corresponds to said first piezoelectric segment to be removed and
said boundary region, etching said piezoelectric block in said
boundary region to reach said sacrificial layer, and eliminating
said sacrificial layer to remove said first piezoelectric
segment.
2. The method according to claim 1, wherein said piezoelectric
block is etched by means of plasma etching.
3. The method according to claim 1, wherein said sacrificial layer
is eliminated by means of etching.
4. The method according to claim 3, wherein said sacrificial layer
is formed of Si, and said etching is performed using XeF.sub.2.
5. The method according to claim 1, wherein said sacrificial layer
is formed of an organic material.
6. The method according to claim 1, wherein a width of said mask
aperture is 20 .mu.m or less.
7. The method according to claim 1, wherein said piezoelectric
block consists of three or more kinds of elements.
8. The method according to claim 7, wherein said piezoelectric
block consists of a lead-containing piezoelectric single crystal or
lead-containing piezoelectric ceramics, or a potassium niobate.
9. The method according to claim 1, which further comprises a step
of forming an etch-stop layer on the opposite surface of said
piezoelectric block having said sacrificial layer.
10. A method of manufacturing a composite piezoelectric substance,
which comprises the steps of; forming, on one surface of a
piezoelectric block, an etching mask having an aperture which
defines a boundary region between a first piezoelectric segment to
be removed, and a second piezoelectric segment to be left remained,
forming, on the opposite surface of said piezoelectric block, an
sacrificial layer which corresponds to said first piezoelectric
segment to be removed and said boundary region, etching said
piezoelectric block in said boundary region to reach said
sacrificial layer, and eliminating said sacrificial layer to remove
said first piezoelectric segment, filling at least partially a
space formed by the removal of said first piezoelectric segment
with an organic material, and forming, on said one surface and said
opposite surface of said piezoelectric block, electrodes for
driving said second piezoelectric segment left remained.
11. The method according to claim 10, wherein said piezoelectric
block is etched by means of plasma etching.
12. The method according to claim 10, wherein said sacrificial
layer is eliminated by means of etching.
13. The method according to claim 12, wherein said sacrificial
layer is formed of Si, and said etching is performed using
XeF.sub.2.
14. The method according to claim 10, wherein said sacrificial
layer is formed of an organic material.
15. The method according to claim 10, wherein a width of said mask
aperture is 20 .mu.m or less.
16. The method according to claim 10, wherein said piezoelectric
block consists of three or more kinds of elements.
17. The method according to claim 16, wherein said piezoelectric
block consists of a lead-containing piezoelectric single crystal or
lead-containing piezoelectric ceramics, or a potassium niobate.
18. The method according to claim 10, which further comprises a
step of forming an etch-stop layer on the opposite surface of said
piezoelectric block having said sacrificial layer.
19. The method according to claim 18, which further comprises a
step of bonding a supporting substrate to the opposite surface of
said piezoelectric block having said etch-stop layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.
2000-146630, filed May 18, 2000, the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a method of manufacturing a
composite piezoelectric substance adapted for use in an ultrasonic
probe, etc., and also to a method of working a piezoelectric
substance to be employed in the manufacture of a composite
piezoelectric substance.
[0003] As shown in "Hand Book of Ultrasonic Diagnostic Equipments,
Revised Edition; Nippon Electronic Mechanical Industries
Association; Corona Publishing Co., Ltd., 1997, 1/20, p68-74", the
ultrasonic probe is constructed such that it is mainly consisted of
a piezoelectric element, an acoustic matching layer, an acoustic
lens and a backing material, all of which are integrated with each
other. The piezoelectric element is formed of a piezoelectric
ceramic board with electrodes on the opposite surfaces thereof. The
acoustic matching layer and the acoustic lens are formed on a side
of the piezoelectric element where an ultrasonic wave is
transmitted or received. The backing material is formed on the back
side of the piezoelectric element.
[0004] This ultrasonic probe is operated as follows. A driving
pulse of one hundred to several hundreds volts is transmitted from
a pulser to the piezoelectric element. Due to this driving pulse,
the piezoelectric element is suddenly deformed by a reverse
piezoelectric effect. As a result of this deformation, the
ultrasonic pulse is excited and emitted via the acoustic matching
layer and the acoustic lens.
[0005] The ultrasonic pulse thus oscillated is then reflected by an
object. The ultrasonic pulse thus reflected re-enters via the
acoustic lens and the acoustic matching layer into the
piezoelectric element, thus mechanically vibrating the element.
This mechanical vibration of the element is converted by way of
piezoelectric effect into an electric signal which is then
transmitted to a monitoring device and reproduced as an image. The
object which reflects the ultrasonic pulse is an interface between
tissues of one's body when the pulse is employed for a medical
purpose. The object is a discontinuous portion in a measuring
object, such as flaw, when the pulse is employed for a
non-destructive examination.
[0006] Generally, a piezoelectric ceramic is employed for the
manufacture of the piezoelectric element of the ultrasonic probe.
In recent years, however, a composite piezoelectric transducer
using a composite piezoelectric substance consisting of a
piezoelectric rod (rod-shaped piezoelectric ceramics) and a resin
has been actually utilized as an electromechanical energy
transducer for the piezoelectric element.
[0007] As set forth in Japanese Patent Unexamined Publication
S60-85699 (hereinafter, referred to as a prior art 1), one example
of the conventional method of manufacturing the composite
piezoelectric substance involves a step of dicing a block-shaped
piezoelectric substance (piezoelectric block) to manufacture
piezoelectric rods. This method is performed as follows. First of
all, a piezoelectric block of, for example, lead zirconate titanate
(PZT) is adhered to a substrate with an adhesive. Then, the
piezoelectric block on the substrate is diced with a dicing device
to obtain a plurality of piezoelectric rods. The groove portion
formed as a result of this dicing is filled with a resin such as
epoxy resin or urethane resin, and then, the resin is allowed to
cure. Finally, the piezoelectric block diced in this manner is
removed from the substrate to obtain a composite piezoelectric
substance.
[0008] More specifically, above mentioned manufacturing method can
be performed by two different procedures. One of the procedures
involves the steps of completely cutting the piezoelectric block by
way of dicing, filling the groove with a resin, curing the resin,
and removing the piezoelectric block from the substrate to obtain
the composite piezoelectric substance. The other procedure involves
the steps of dicing incompletely the piezoelectric block, filling
the groove portion with a resin, curing the resin, removing the
piezoelectric block from the substrate, and grinding or slicing the
piezoelectric block to obtain the composite piezoelectric
substance.
[0009] Another example of manufacturing method of the composite
piezoelectric substance is set forth in "Jpn. J. Appl. Phys."; Vol.
36(1997), pp. 6062-6064 (hereinafter, referred to as a prior art
2). This prior art discloses a method of manufacturing a composite
piezoelectric transducer, wherein a deep X-ray lithography and a
resin molding method are combined to obtain piezoelectric rods
having a high aspect ratio, and then, the space formed between the
piezoelectric rods is filled with a resin to obtain the composite
piezoelectric transducer.
[0010] More specifically, first of all, a resist film having a
thickness of 400 .mu.m and consisting of MMA (methyl
methacrylate)/MAA (methacrylic acid) copolymer is deposited on a
substrate. Then, a synchrotron radiation is irradiated via a mask
onto the resist film, which is followed by the development of the
resist film (deep etch X-ray lithography), thereby obtaining a
resist structure having a plurality of apertures. Thereafter, a PZT
(lead zirconate titanate) slurry is poured into these apertures of
the resist structure. The pouring of the PZT slurry, which is
consisted of a PZT powder, a binder and water, is performed
utilizing the resist structure as a resin mold.
[0011] Then, the PZT slurry is allowed to dry and cure at a room
temperature to obtain a PZT green body. Then, only the resin mold
is removed by means of an oxygen plasma, thus leaving the PZT green
body. The PZT green body thus left is next subjected to a defatting
treatment (the removal of the binder) at a temperature of
500.degree. C., and then to a main sintering treatment at a
temperature of 1,200.degree. C. As a result of the sintering, a PZT
rod array consisting of a plurality of PZT rods (piezoelectric
columns) each having a dimension of 20 .mu.m in diameter and 140
.mu.m in height is obtained.
[0012] Then, the space between the PZT rods is filled with epoxy
resin by means of vacuum impregnation, which is followed by the
curing of the epoxy resin to obtain a composite piezoelectric
substance. Thereafter, the top and bottom surfaces of the rod array
are polished until the opposite end faces of the PZT rods are
exposed. Thereafter, gold electrodes are deposited by means of
sputtering on the flattened top and bottom surfaces of the rod
array. Thereafter, the rod array is subjected to a poling treatment
by impressing a voltage onto the electrodes under the condition
wherein the rod array is kept immersed in an oil bath. As a result,
a small/thin type composite piezoelectric transducer constituted by
a composite piezoelectric substance having a piezoelectric property
can be obtained.
[0013] However, the aforementioned conventional methods of
manufacturing the piezoelectric structure are accompanied with the
following problems.
[0014] (1) In the method set forth in the prior art 1, if the
diameter of the piezoelectric rod is made too small by way of the
dicing, the piezoelectric rod may be easily be destroyed during
dicing. Therefore, it has been very difficult to obtain a
piezoelectric rods of a small diameter, for instance 50 .mu.m or
less, so that it has been difficult to manufacture piezoelectric
rods of a high aspect ratio. As a result, it has been difficult to
manufacture a composite piezoelectric transducer exhibiting a very
high oscillating frequency which is required for improving a
resolution of a diagnostic apparatus. Further, the piezoelectric
rods manufactured by dicing work can only have a configuration of a
polygonal cross-section and walls extending straight along a
longitudinal axis thereof. Therefore, a redundant oscillation mode
in a direction perpendicular to the longitudinal axis can arise in
the vicinity of the resonance frequency in the direction of the
longitudinal axis of piezoelectric rod, thereby causing noises in
the ultrasonic probe.
[0015] (2) In the method set forth in the prior art 2, falling-down
of the piezoelectric rods during sintering makes it difficult to
reduce the diameter of the rods to less than several tens
micrometers. Therefore, it has been very difficult to increase the
aspect ratio of the rods and therefore to obtain a composite
piezoelectric oscillator exhibiting a very high oscillation
frequency.
[0016] Moreover, the resin mold makes it impossible to utilize a
hot press or a hot isostatic press (HIP). Therefore, it has been
impossible to highly densify the piezoelectric substance in order
to sufficiently utilize its excellent intrinsic properties.
Additionally, it is substantially impossible to apply the method in
the prior art 2 to a piezoelectric single crystal.
BRIEF SUMMARY OF THE INVENTION
[0017] Therefore, the object of the invention is to provide a
method of working a piezoelectric substance to obtain a
piezoelectric structure, which is possible to finely work the
substance and is also applicable to a piezoelectric single crystal,
and a method of manufacturing a composite piezoelectric substance
using the piezoelectric structure.
[0018] Accordingly, this invention provides a method of working a
piezoelectric substance, which comprises the steps of;
[0019] forming, on one surface of a piezoelectric block, an etching
mask having an aperture which defines a boundary region between a
first piezoelectric segment to be removed, and a second
piezoelectric segment to be left remained,
[0020] forming, on the opposite surface of the piezoelectric block,
a sacrificial layer which corresponds to the first piezoelectric
segment to be removed and the boundary region,
[0021] etching the piezoelectric block in the boundary region to
reach the sacrificial layer, and
[0022] eliminating the sacrificial layer to remove the first
piezoelectric segment.
[0023] This invention also provides a method of manufacturing a
composite piezoelectric substance, which comprises the steps
of;
[0024] forming, on one surface of a piezoelectric block, an etching
mask having an aperture which defines a boundary region between a
first piezoelectric segment to be removed, and a second
piezoelectric segment to be left remained,
[0025] forming, on the opposite surface of the piezoelectric block,
a sacrificial layer which corresponds to the first piezoelectric
segment to be removed and the boundary region,
[0026] etching the piezoelectric block in the boundary region to
reach the sacrificial layer,
[0027] eliminating the sacrificial layer to remove the first
piezoelectric segment,
[0028] filling at least partially a space formed by the removal of
the first piezoelectric segment with an organic material, and
[0029] forming, on the one surface and the opposite surface of the
piezoelectric block, electrodes for driving the second
piezoelectric segment left remained.
[0030] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0031] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0032] FIGS. 1A to 1M represent cross-sectional views illustrating
in stepwise one example of the method of manufacturing a
piezoelectric structure and a composite piezoelectric transducer
according to this invention;
[0033] FIG. 2 is a schematic view showing one example of the
ICP-RIE device according to this invention;
[0034] FIG. 3 is a graph showing one example of the dependency of a
sidewall taper angles on the make aperture width according to this
invention;
[0035] FIG. 4 illustrates a perspective view schematically showing
one example of a composite piezoelectric transducer according to
this invention;
[0036] FIG. 5 is a perspective view schematically showing one
example of a distal end portion of an ultrasonic probe according to
this invention;
[0037] FIGS. 6A to 6D respectively shows another example of the
method of manufacturing a piezoelectric structure and a composite
piezoelectric transducer according to this invention;
[0038] FIG. 7 shows a perspective view schematically showing
another example of a composite piezoelectric transducer according
to this invention;
[0039] FIGS. 8A and 8B are plan views schematically illustrating
one example of the piezoelectric substance on which a mask is
formed according to this invention;
[0040] FIGS. 9A and 9B are perspective views schematically
illustrating, respectively, one example of a composite
piezoelectric substance of 2-2 structure and one example of cutting
positions to form an array of piezoelectric rods according to this
invention; and
[0041] FIGS. 10A and 10B are cross-sectional views schematically
illustrating one example of the composite piezoelectric substance
according to this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0042] This invention will be further explained in detail with
reference to drawings.
[0043] On manufacture of a piezoelectric structure, an etching mask
is formed on one surface of a piezoelectric block at first. The
mask pattern includes a narrow aperture for partitioning the
piezoelectric block into a first piezoelectric segment which is
useful, and a second piezoelectric segment which is redundant. The
useful segment in this case means a segment which should be left
remained as piezoelectric columns, etc. Whereas the redundant
segment means a segment which should be removed in order to form a
space. In other words, the mask aperture defines a boundary region
between the first piezoelectric segment to be removed, and the
second piezoelectric segment to be left remained. On the opposite
surface of the piezoelectric block, there is formed one or more
sacrificial layers. The sacrificial layers are formed so as to
correspond to the redundant segments as well as the narrow boundary
regions defining the redundant segments. The mask apertures, which
define the boundary regions, function to distinguish the redundant
segments from the surrounding useful segments. After the
sacrificial layers are formed as described above, an etch-stop
layer is formed over the opposite surface of the piezoelectric
block. The etch-stop layer is formed from a material which can be
hardly etched, such as nickel. Further, a supporting substrate is
bonded to the etch-stop layer. This supporting substrate is
designed to support the etched piezoelectric block. The regions of
the piezoelectric block exposed within the mask apertures (boundary
regions) are etched with a plasma, for example, so as to divide the
piezoelectric block into useful segments and redundant segments.
This etching is performed so as to form through-holes passing
through the piezoelectric block until reaching the sacrificial
layer. The redundant segments of the piezoelectric block are
mounted on the sacrificial layers. Finally, the sacrificial layers
are eliminated by means of etching or by making use of a releasing
solution, to remove the redundant segments together with the
sacrificial layers. Thus, the piezoelectric structure is
obtained.
[0044] Being different from a piezoelectric material of thin film
deposited on a substrate, the piezoelectric block is solely
manufactured from a piezoelectric material through molding and
sintering steps. The piezoelectric block can be formed from a
piezoelectric ceramic material. The piezoelectric ceramic materials
exhibiting excellent piezoelectric properties include a
piezoelectric ceramic material and a piezoelectric single crystal
which respectively consists of at least three kinds of element. In
particular, piezoelectric materials containing lead element are
especially preferable.
[0045] Examples of such piezoelectric materials include
piezoelectric ceramics such as lead zirconate titanate (PZT type)
Pb(Zr, Ti)O.sub.3, lead magnesium niobate-lead titanate (PMN-PT
type) {Pb(Mg, Nb)O.sub.3--PbTiO.sub.3}, lead zinc niobate-lead
titanate (PZN-PT type) {Pb(Zn, Nb)O.sub.3--PbTiO.sub.3}, and single
crystals such as a single crystal of PZN-PT type solid solution, a
PMN-PT type single crystal, or a potassium niobate (KNbO.sub.3)
plate-shaped single crystal cut out at an angle of 44 degrees
relative to the polarization axis.
[0046] If the potassium niobate single crystal is worked into a
rod-shaped piezoelectric substance, it exhibits excellent
piezoelectric properties. Such a piezoelectric single crystal can
provide a composite piezoelectric transducer exhibiting a very
excellent piezoelectric properties, resulting in that the S/N ratio
of the final ultrasonic probe can be increased, thereby achieving a
high resolvability. Since the piezoelectric single crystal is very
brittle depending on the combination between the stress to be
applied and the crystal orientation, it is very difficult to
suitably work the piezoelectric single crystal. However, if an
optimized etching condition is selected, the piezoelectric single
crystal can be etched at a high speed to manufacture a composite
piezoelectric transducer consisting of fine piezoelectric columns,
etc. The etching condition can be optimized by suitably selecting a
reactive gas which enables a reaction product to be easily
evaporated and is capable of providing a large bombardment. It is
possible, by making use of the same procedures in this manner, to
perform a high-speed etching of not only the piezoelectric single
crystal but also the aforementioned various materials.
[0047] The etching mask to be deposited on the piezoelectric block
can be deposited using any kind of material and any film-forming
method which can realize a smaller selectivity ratio of the mask
during plasma etching than that of the piezoelectric material (i.e.
the etching rate of the mask is smaller than that of the
piezoelectric material). For example, materials deposited by CVD
such as silicon oxide (SiO.sub.2), silicon nitride
(Si.sub.3N.sub.4) and calcium fluoride (CaF.sub.2); materials
deposited by electrolytic plating such as nickel (Ni) and copper
(Cu); and a resinous resist itself can be used.
[0048] The etching mask may be formed in such a manner that a
conductive mask such as Ni is deposited at first and then, a
material which can be easily charged up such as quartz (SiO.sub.2)
is deposited up to several hundreds nanometers on the conductive
mask by means of sputtering for instance. When the etching mask is
formed in this manner, the mask is charged up during the plasma
etching thereof, thereby causing the incoming ions to deflect so as
to make it possible to effectively adjust the profile of etching
surface.
[0049] It is also possible to prepare a stencil mask in advance so
as to make it available as an etching mask. In this case, since
good or bad finishing of etching can be determined by judging only
the features of the mask, it is possible to improve the yield.
[0050] As for the configuration of the piezoelectric column, etc.
to be fabricated by the etching, it may be hexagonal or any other
polygonal configuration in cross-section other than the columnar
configuration. Further, the layout of the piezoelectric columns may
not be regular. The distance between neighboring piezoelectric
columns should better be non-uniform in view of alleviating the
oscillation in the horizontal direction which may become a cause
for the noise.
[0051] The mask pattern can be determined depending on the
configuration and dimension of the piezoelectric columns to be
manufactured. For example, the mask pattern can have ring-shaped
apertures, each having the same diameter as that of each of the
columnar piezoelectric rods to be left after etching. The dimension
of mask pattern, such as the ring diameter, depends on the diameter
of the columnar piezoelectric rod.
[0052] In view of facilitating the removal of the redundant
segments of the piezoelectric block, it is preferable to form slits
having a width of several micrometers in the mask pattern so as to
partition each segment into a plurality of sub-segments, thereby
improving the yield.
[0053] The width of the mask aperture (where the piezoelectric
substance is exposed and etched) should preferably be 20 .mu.m or
less. Generally, it is difficult to perpendicularly etch the
piezoelectric substance, the sidewall of the etched piezoelectric
column etc. being tapered. However, if the width of the mask
aperture is narrowed as described above, the piezoelectric
substance can be approximately perpendicularly etched (this
perpendicular etching differs more or less for different kinds of
piezoelectric material, depending on etching conditions such as the
kind of reactive gas, the pressure of chamber during the etching,
the etching temperature, as well as the volatility of reaction
products during etching). The taper angles of the sidewall after
etching largely depends on the mask aperture width, so that if the
mask aperture with is narrowed suitably, the taper angle tends to
become 90 degrees, thus making it possible to perform a
perpendicular etching. This dependency on the mask aperture width
can be attributed to the volatility of reaction products during
etching and to the cohesive strength of atoms or molecules of
piezoelectric materials. Namely, the lower the volatility of
materials in the piezoelectric material is, the narrower the
aperture width should be in order to perform a perpendicular
etching. A perpendicular etching can keep the etch width narrow,
thereby forming a piezoelectric column of a high aspect ratio.
[0054] More specifically, it is preferable that the width of
aperture of mask is restricted to not more than 10 .mu.m in
addition to the optimization of etching conditions on the occasion
of etching a piezoelectric material which tends to be tapered by
the etching. If, for instance, the width of aperture of mask is set
to 3 .mu.m, almost all kinds of piezoelectric material can be
perpendicularly etched.
[0055] In particular, when the piezoelectric material consists of
three or more kinds of elements as described above, the kinds of
reaction products would increase in proportion to an increase in
kind of the constituent elements, thereby making it more difficult
to perform a perpendicular etching. If it is desired to obtain a
desired fine piezoelectric structure from a piezoelectric block
consisting of three or more kinds of elements, the peripheral
portion of each of the redundant segments of piezoelectric block is
preferably perpendicularly etched away through the aforementioned
mask apertures of narrow width followed by removing the redundant
segments.
[0056] For example, when the piezoelectric material is formed of
KNbO.sub.3, the width of mask aperture should preferably be limited
to the range of not more than several micrometers to not more than
ten-odd micrometers. On the other hand, when the piezoelectric
material is formed of PZT type ceramics, the width of mask aperture
should preferably be limited to the range of 2 to 3 micrometers or
less. Further, if quartz crystal is to be etched according to the
method of this invention, the width of mask aperture should
preferably be limited to not more than 20 micrometers.
[0057] Since the angle of tapering of sidewall would become larger
as the width of mask aperture is increased, if it is desired to
provide the sidewall of the etched piezoelectric column with a
suitable angle of tapering, the width of mask aperture can be
increased in conformity with the configuration of the desired
piezoelectric column.
[0058] When the etching is performed under the condition where the
width of mask aperture is sufficiently narrowed, it is not only
possible to execute a perpendicular etching to thereby obtain a
piezoelectric column of high aspect ratio, but also possible to
reduce the quantity to be etched of piezoelectric block. As a
result, the deposit of the reaction products in the etching
apparatus can be reduced to thereby facilitate the maintenance of
the etching apparatus. Since a heavy metal such as lead is
generally included in the piezoelectric material and halogen is
included in the etching gas, a reaction product is inevitably
included in the exhaust gas discharged from the etching apparatus.
However, since the quantity of the piezoelectric block to be etched
away is minimized, the content of the reaction products in the
exhaust gas and the use of the etching gas can be minimized. This
is important in respect of preserving the environments.
[0059] Further, in the case of providing different sizes of spaces
between the piezoelectric columns, a micro-loading effect during
etching (the effect that the etching rate is dependent on the width
of etching) can be extremely minimized. This may be attributed to
the fact that only a narrow through-groove is formed by etching at
the periphery of each segment to be removed to provide spaces, so
that the width of etching is constantly kept narrow for different
sizes of the spaces between columns. Furthermore, the etch-stop
layer (which will be explained hereinafter) can be formed to
correspond only to the segments to be removed and their peripheral
through-holes (in the same way as the sacrificial layer) without
corresponding to the piezoelectric columns. This results in a
minimum size of the etch-stop layer, making the process
efficient.
[0060] The aforementioned technique of perpendicular etching of the
piezoelectric substance in the mask aperture of narrow width can be
applied not only to a block-shaped piezoelectric material but also
to a thin-film piezoelectric material.
[0061] After an etching mask is formed as described above, one or
more sacrificial layers consisting of silicon for instance is
formed on the rear surface of the piezoelectric block (the lower
surface of the block). The sacrificial layers are deposited to
correspond to the redundant segments of the piezoelectric substance
to be removed as well as the narrow mask apertures distinguishing
the redundant segments from the other necessary segments. The
sacrificial layers are not confined to silicon but may be any kind
of materials removable without a damage to the piezoelectric
substance. For example, the layer can be made from an organic
material such as a photoresist which can be removed with an organic
solvent and is compatible with a sputtering and plating.
[0062] After the deposition of the sacrificial layer, an etch-stop
layer is deposited by sputtering on the lower surface of the
piezoelectric block where the sacrificial layer has been deposited.
This etch-stop layer is formed of any materials which can be hardly
etched away during plasma etching. For example, nickel or any other
kinds of material which can be utilized as an etching mask material
can be employed. In the same way as in the etching mask, a material
which is capable of exhibiting a smaller selectivity ratio as
compared with the piezoelectric material during plasma etching may
be employed. The etch-stop layer can correct any differences in
etching rate due to a non-uniformity of etching which may be caused
by an etching apparatus. The method of depositing the etch-stop
layer is not confined to the sputtering method, but various kinds
of vapor deposition method or the same procedures as applicable to
the deposition of the etching mask as mentioned above can be
employed.
[0063] A supporting substrate is then bonded to the lower surface
of the piezoelectric block provided in advance with the etch-stop
layer. For example, a supporting substrate made of the same kind of
ceramics as employed for the piezoelectric block can be bonded to
the lower surface with a bonding agent such as an indium-based
alloy which can be fused at a relatively low temperature. The
material for the supporting substrate can be any kinds of material
which are incapable of raising any problem during plasma etching,
it is preferable to employ the same kind of material as that of the
piezoelectric block. This can minimize the generation of stress
because a less difference in thermal expansion coefficient occurs
between the piezoelectric substance and the supporting substrate
during plasma-etching at the elevated temperature of the block. As
a result, the possibility of peeling at the bonded portion between
the piezoelectric substance and the supporting substrate can be
minimized. The etch-stop layer can be formed of other kinds of
material exhibiting almost the same level of thermal expansion
coefficient as that of the piezoelectric substance, such as a 42
alloy or glass of low thermal expansion coefficient.
[0064] After the supporting substrate is assembled in this manner,
the regions of the piezoelectric substance exposed within the mask
aperture are etched away by plasma for instance, down to a depth
reaching the sacrificial layer, thereby forming through-holes. The
through-holes can separate the block into two groups of segments,
one group to be left remained, and another group to be removed.
[0065] The etching method using plasma is well known as a dry
etching method in the field of Si processing. Unlike the materials
such as Si and SiO.sub.2 employed in the Si processing, the
piezoelectric material contains components which result in a
reaction product which can be hardly volatilized. Suitable
selection of reactive gas is very important to enhance the etching
rate and to improve the profile of the sidewall. Specifically,
gases (halogen including chlorine) which are capable of generating
a reaction product of a high vapor pressure through a reaction with
a constituent element of the piezoelectric substance is very
effective as a reactive gas in this regard.
[0066] More specifically, the etching can be performed by means of
a reactive ion etching method using an inductively coupled plasma
(ICP-RIE), or a parallel plate reactive ion etching method.
[0067] The reactive gases to be employed in the etching can be
sulfur hexafluoride (SF.sub.6) gas, or a mixed gas comprising
SF.sub.6 gas and at least one additional gas selected from the
group consisting of oxygen and inert gases (argon (Ar), krypton
(Kr), xenon (Xe), etc.). It is also preferable to employ a
chlorine-based gas such as a single chlorine gas, a mixed gas
consisting of a chlorine-based gas and a fluorine-based gas such as
carbon fluoride (CF.sub.4), or a mixed gas consisting of a
chlorine-based gas and a gas containing a substance (Kr, Xe, etc.)
which is capable of giving an ion of large molecular weight or
large atomic weight in the plasma. The reason is that these gases
can provide a large etching rate and can realize an easy control of
the angle of tapering of sidewall of the etched piezoelectric
column. Namely, chlorine or fluorine in the gas contribute to the
chemical etching, whereas the ion having a large molecular weight
or a large atomic weight in the plasma contributes to the physical
etching. More specifically, when the self-bias is increased, the
ion of this kind can increase the bombardment speed or the energy
of the bombardment against the piezoelectric block of a material to
be etched, then the physical etching being promoted.
[0068] When an inert gas is mixed into an SF.sub.6 gas or a
chlorine-based gas, the plasma can be stabilized. In particular, Kr
and Xe can contribute to the stabilization of plasma under a low
pressure in addition to the contribution to the physical
etching.
[0069] These etching gases and a gas (such as C.sub.4F.sub.8) which
can form a deposit on the etching surface may be alternately
introduced to be formed into a plasma, thereby repeating the
etching and the deposition so as to control the taper angles.
[0070] When a chlorine-based gas is to be employed as an etching
gas, the etching apparatus is required to be constituted by a
material which is excellent in corrosion resistance.
[0071] The etching work of the piezoelectric block should
preferably be performed in a pressure of not more than 20 mTorr.
This is because lower pressure can decrease the number of
bombardments between the ions and other kinds of molecules, thereby
enhancing the physical etching rate. Furthermore, lower pressure
can volatilize the reaction products more easily during etching
because of the reaction between the ions or radicals and the
piezoelectric substance, thereby enhancing the etching rate.
[0072] The temperature of specimens during etching should
preferably be higher than room temperature in view of enhancing the
productivity. Since the actual piezoelectric substance can include
at least three kinds of additives such as Mn, Co, Ni, W, etc., the
reaction products generated during etching can include various
kinds of material. If the piezoelectric substance includes
additives which can result in reaction products of low volatility,
such reaction products will deposit during an etching at a low
temperature, thereby obstructing the high-speed etching. The
etching at a temperature higher than room temperature can suppress
the deposition of reaction products, thereby increasing the etching
rate and realizing a perpendicular etching relative to the main
surface of the piezoelectric block. For example, in the case of the
PZT where many kinds of additives are included therein, the
temperature of the specimens should preferably be controlled in the
range of 50 to 300.degree. C.
[0073] The plasma density should preferably be as high as possible
for the purpose of achieving a high-speed etching work. For
example, the high-frequency, power applied to a plasma-generating
coil should be higher than 100 W. For the purpose of enhancing the
plasma density, the gap between the stage and the inner wall (for
example, a quartz plate) of reaction vessel facing the stage should
be as narrow as possible (specifically, 50 mm or less).
[0074] The AC voltage applied to the stage should preferably be as
high as possible to enhance the etching rate so as to improve the
productivity. Specifically, the AC voltage should preferably be
applied to make a self bias not higher than -300V. Even if the
power source for the stage is a high-frequency power source (13.56
MHz), which is the same as the power source for the coil, in place
of the low frequency power source of several hundreds KHz, the
etching can be performed in almost the same manner. A lower
frequency voltage can preferably be applied to the stage to make
the slower switching between the positive and negative voltages of
the stage, thereby increasing the ion bombardment. As a result, the
contribution of physical etching becomes larger, thereby enhancing
the etching rate as compared with the case where a high-frequency
power source is employed. Additionally, since the quantity of
deposit can be minimized, it becomes possible to obtain a more
excellent etching profile. Furthermore, use of a low frequency
power source for the power source for the stage can result in
repetitions between an etching with negative ions and an etching
with positive ions, thereby further enhancing the etching rate.
[0075] For the purpose of controlling the configuration of the
sidewall after the etching, the high-frequency voltage should
preferably intermittently applied to the sample mounting stage. The
intermittent application of the high-frequency voltage can improve
the angle of the sidewall, which may vary depending on the
piezoelectric materials, without interrupting the plasma. This is
especially effective in the case where the insulative piezoelectric
substance is charged up, and preventing the ions from being
perpendicular incident on the piezoelectric substance. Further, an
intermittent application of the voltage to the stage facilitates
the reaction products to be discharged from the chamber to prevent
them being accumulated therein, thereby making it possible to
reduce the quantity of the deposit to be formed inside the
chamber.
[0076] A high-frequency voltage may be applied to the stage which
is already applied a positive voltage, thereby keeping the stage
positive and performing the etching utilizing negative ions. Use of
negative ions consisting mainly of halogens can perform an etching
of an enhanced selectivity to the mask, even though the etching
rate may be somewhat sacrificed.
[0077] The etching may be performed with varied etching conditions
so as to provide the sidewalls of the piezoelectric substance with
suitable taper angles. The tapered sidewall can minimized the
redundant oscillation mode in the radial direction of the
piezoelectric column, thereby making it possible to manufacture a
probe having an improved S/N ratio and an improved sensitivity.
Furthermore, the tapered sidewall can alter only the array pitch of
the piezoelectric substances (with the different sizes of the resin
portion).
[0078] The next step is, as described above, to eliminate the
sacrificial layer after the plasma etching. Together with the
sacrificial layer, the redundant segments of piezoelectric
substance mounted on the sacrificial layer are removed to leave the
piezoelectric columns of desired configuration, thus manufacturing
a piezoelectric structure. The spaces formed after the removal of
the piezoelectric substance are filled with a resin as explained
hereinafter.
[0079] Where the sacrificial layer consists of Si, the etching
thereof can be performed by making use of XeF.sub.2 (xenon
fluoride). Other than the etching method using XeF.sub.2, any kinds
of etching method can be employed which are capable of etching Si
without any substantial damage to the piezoelectric substance. When
the etching method using XeF.sub.2 is employed, the etching rate
for materials other than Si is much slower than that for Si. Even
the ordinary epoxy resin cannot be substantially etched away by
XeF.sub.2, so that only Si can be etched away by XF.sub.2 with
excellent selectivity. As described above, when an organic material
such as a resist is employed for the sacrificial layer, the
sacrificial layer can be removed by immersing it in an organic
solvent such as acetone or in a releasing agent dedicated for the
resist, or by exposing it to an oxygen plasma.
[0080] After the piezoelectric structure is manufactured by
removing the sacrificial layer with redundant segments of the
piezoelectric substance, the spaces such as grooves or holes formed
by the removal of the redundant segments of the substance are
filled with organic materials. The organic materials can be any
ones which have a high adhesive strength to the piezoelectric
columns, although flexible organic materials are preferable.
Examples of such flexible organic materials include epoxy resin,
urethane resin, silicone resin.
[0081] Even if the spaces are not completely filled with a resin,
spaces partially filled with a resin can also provide the
characteristics of a composite piezoelectric transducer. The air
left in the spaces where the resin is not filled can minimized the
cross-talk between the neighboring piezoelectric rods, and can
lower the acoustic impedance to improve the permeability of the
ultrasound into the human body. Thus, the picture quality in an
ultrasonic diagnostic apparatus is expected to be improved.
[0082] After the filling of the resin in the spaces, the electrodes
for driving the piezoelectric columns are formed on the upper and
lower surfaces of the piezoelectric block. The electrode material
can be any material which will not give any substantial damage to
the organic materials during the electrodes formation. Examples of
such an electrode material include metals such as gold, copper,
titanium, nickel, silver, platinum, chromium, and combination
thereof such as a laminated metal of a chromium/gold, for example,
and compound materials such as ITO (indium tin oxide). The method
of forming the electrodes can be sputtering, vapor deposition, ion
plating.
[0083] As a final step, the external profile of the specimen is
worked to accomplish the composite piezoelectric transducer.
[0084] According to the working method of the piezoelectric
structure of this invention, a block-shaped piezoelectric substance
exhibiting excellent piezoelectric properties is worked, so that it
is possible to obtain a piezoelectric column of high aspect ratio,
which is excellent in piezoelectric properties and has a fine and
high-precision configuration. The piezoelectric column of a fine
and high-precision configuration can allow the composite
piezoelectric transducer to be easily miniaturized. Furthermore,
the aspect ratio of the piezoelectric column is sufficiently high,
so that the transmitting and receiving properties of the composite
piezoelectric transducer can be improved, whereby the S/N ratio of
transmitting and receiving signals can be enhanced.
[0085] Since the piezoelectric structure is fabricated by means of
dry etching method, the inclusion of oily components or
contaminants into the structure can be extremely minimized as
compared with the case where the piezoelectric structure is
fabricated by means of mechanical working, thereby making it
possible to improve the adhesivity between the resin and the
piezoelectric columns.
[0086] Further, since the piezoelectric block is subjected to fine
working by means of dry etching, a single crystal piezoelectric
substance (which is difficult to be subjected to a mechanical
working such as dicing and is impossible to be worked with a lost
mold process utilizing LIGA etc.) can be easily worked.
[0087] As explained above, the etching conditions can be adjusted
to provide the sidewall of the piezoelectric column with suitable
taper angles. As a result, the redundant vibration mode in the
radial direction of the piezoelectric column can be minimized as
mentioned above, so that it is possible to manufacture a probe
having an improved S/N ratio and an enhanced sensitivity.
[0088] Next, a first embodiment of the method according to this
invention will be explained with reference to FIGS. 1A to 1M.
[0089] The manufacturing method according to this embodiment
comprises steps of: (1) manufacturing a piezoelectric block; (2)
forming a mask on the piezoelectric block; (3) etching the
piezoelectric block; (4) removing redundant elements of
piezoelectric block; (5) filling an organic material; and (6)
polishing, providing electrodes and providing piezoelectricity.
[0090] (1) A step of manufacturing a piezoelectric block:
[0091] A block-shaped piezoelectric substance (piezoelectric block)
is prepared at first by means of ordinary method. For example, a
mixed powder of a desired composition is preliminarily baked,
pulverized, mixed with a binder in order to improve the
moldability, graded to obtain a graded mixture, and molded into
pellets by means of pressing method. The pellets are then heated at
a temperature of 600.degree. C. to remove organic substances
remaining therein. Thereafter, the pellets are subjected to a hot
press at a temperature of 1,200.degree. C. to obtain a sintered
body. The sintered body is then shaped using a slicer, a surface
grinder, and, if required, a double lapping machine thereby to
obtain a piezoelectric block 25 as shown in FIG. 1A, which is
subsequently subjected to the etching work. If a PZT-based material
is employed for example as a piezoelectric material, it is possible
to employ a material having frequency constants of: Nt=about 2000
Hz-m, and N33=about 1300 Hz-m. In this case, if the thickness of
the piezoelectric block 25 is set to 0.1 mm, the resonance
frequency of rod vibration to which N33 is applicable would become
13 MHz.
[0092] (2) A step of forming a mask on the piezoelectric block:
[0093] As shown in FIG. 1A, chromium (Cr) and gold (Au) are
successively deposited by means of sputtering on the surface of the
piezoelectric block 25 to thereby form a thin film electrode
20.
[0094] Then, as shown in FIG. 1B, a photoresist layer is coated on
the Cr/Au electrode 20 to form a resist layer 21.
[0095] As shown in FIG. 1C, the photoresist layer 21 is subjected
to a patterning exposure so as to leave the resist along the
boundaries between the piezoelectric segments to be ultimately left
as piezoelectric columns and the piezoelectric segments to be
removed for filling a resin therein. The photoresist layer 21 is
subsequently subjected to the development step to thereby obtain a
patterned photoresist layer 21. The pattern of the resist layer 21
can include a plurality of ring-shaped resist layers each having a
diameter corresponding to the diameter of the piezoelectric columns
(columnar piezoelectric substances) and a width ranging from 1 to 3
.mu.m.
[0096] Next, as shown in FIG. 1D, the electrolytic plating of
nickel (in a sulfamic acid bath) is performed by applying a
negative voltage to the Cr/Au electrode 20 to thereby form a nickel
film, thus obtaining a pattern of nickel 24.
[0097] Then, as shown in FIG. 1E, the resist layer 21 is removed
using a solvent, thereby obtaining a piezoelectric block 25 bearing
the residual nickel mask 24. This pattern of nickel 24 is formed of
a pattern having ring-shaped apertures 26 having an aperture width
W ranging from 1 to 3 .mu.m in conformity with the ring-shaped
resist 21 left remained as shown in FIG. 1C.
[0098] Then, as shown in FIG. 1F, sacrificial layers 35 consisting
of silicon are formed on the lower surface of the piezoelectric
block in conformity with the piezoelectric segments to be removed
and with the apertures 26 which define boundary regions between
segments to be removed and regions to be left. This silicon layer
35 can be patterned by making use of photolithography. For example,
a resist film is coated on the lower surface of the piezoelectric
block 25 and then, patterned. Then, a silicon layer is formed by
means of sputtering method, and the resist film is removed
(lift-off) to form a pattern of silicon.
[0099] Next, as shown in FIG. 1G, as an etch-stop layer, a nickel
layer 36 which can be hardly etched away during the plasma etching
is formed all over the lower surface of the piezoelectric block by
means of sputtering.
[0100] Finally, as shown in FIG. 1H, a supporting substrate 37
consisting of the same kind of ceramics as that of the
piezoelectric block 25 is bonded to the lower surface of the
piezoelectric block 25 by making use of a bonding agent 38
comprising an indium-based alloy which can be fused at a relatively
low temperature.
[0101] (3) A step of etching the piezoelectric block:
[0102] As shown in FIG. 1I, the piezoelectric block 25 is
perpendicularly plasma-etched according to the mask pattern of the
nickel layer 24 to thereby form holes or grooves 22 in the
piezoelectric block 25. This etching can be performed by way of a
reactive ion etching employing inductively coupled plasma (ICP-RIE)
method wherein SF.sub.6 is employed as a reactive gas.
[0103] FIG. 2 shows a schematic view of a plasma etching apparatus
suited for the etching process. Referring to FIG. 2, a stage 42 for
mounting a sample 43 thereon is disposed inside the aluminum
chamber 41 (if a chlorine-based gas is employed, SUS such as SUS316
which is in corrosion resistance is employed). This stage 42 is
provided with a heater 51 for heating the stage 42 and with a
connecting pipe 56 for passing a cooling medium to the interior of
the stage 42, thereby making it possible to control the temperature
of the stage 42. This connecting pipe 56 is connected with a
coolant-cooling apparatus 57 which is controlled by a temperature
controller 59. Further, the stage 42 is also provided with a
thermocouple 58 which is connected with the temperature controller
59. The stage 42 is electrically insulated from the chamber 41 by
way of an insulator 44, thereby making it possible to apply a
high-frequency voltage to the stage 42. Further, the stage 42 is
connected via a lead wire 48 with an external matching box 52 which
is connected with a low frequency power source 54 (about 700
kHz).
[0104] A lid 45 placed on the top of the chamber 41 is made from
sapphire glass, and a one-turn type inductive coil 46 wherein a
cooling water is allowed to pass therethrough is placed on the lid
45 (on the atmosphere side). A permanent magnet 47 of NbFeB
attached to a magnetic body is centered in the coil 46. This coil
46 is connected via a lead wire 65 with a matching box 60 which is
connected with an RF power source 53 (13.56 MHz) for applying a
high-frequency voltage.
[0105] The reactive gas is uniformly introduced into the chamber 41
through a ring-shaped inlet pipe 49 disposed above the stage 42.
The pressure inside the chamber 41 is maintained constant by means
of a manometer 66 attached to the chamber 41. A high-frequency
current of 13.56 MHz for instance generated from the RF power
source 53 is allowed to pass through the coil 46 so as to cause a
plasma 55 to be generated and maintained thereat. The chamber 41
can be evacuated with a turbo-molecular pump 50 for instance. An
etching apparatus capable of evacuating the chamber 41 at a large
flow rate per unit time can minimize the residence time of the gas,
thereby effectively exhausting the reaction products from the
chamber 41. Therefore, such an etching apparatus would be excellent
in the respects that the etching rate can be accelerated and that
the etching treatment can be performed in large quantities at the
same time.
[0106] As shown in FIGS. 1I and 1J, a deep hole or groove 22 can be
formed by suitably adjusting the etching conditions such as the
temperature of the substrate 43, the pressure inside the chamber
41, the low-frequency output applied to the stage 42 and the
high-frequency output applied to the coil 46.
[0107] As shown in FIG. 1J, the etching is performed down to a
depth reaching the silicon layer 35, thereby forming through-holes
or grooves 22 which penetrates through the piezoelectric block 25.
With the provision of the through-holes or grooves 22, the
piezoelectric block 25 is separated into the segments 61 of the
block to be left and the redundant segments 62 of the block to be
removed. Further, when the pattern of the resist layer 21 shown in
FIG. 1C is changed as desired, the configuration of the
piezoelectric column 61 shown in FIG. 1J can be variously altered
other than the columnar configuration.
[0108] As explained above, the sidewall of the piezoelectric column
61 can be tapered at a desired angle depending on the aperture
width W (shown in FIG. 1E) of the etching mask 24. FIG. 3
represents one example of the dependency of the taper angle on the
width of mask aperture. This dependency varies for different kind
of piezoelectric material depending on etching conditions such as
the kind of reactive gas, the pressure inside the chamber during
the etching, the etching temperature, as well as on the volatility
of reaction products during etching. For example, as shown in FIG.
3, the piezoelectric material A can be substantially
perpendicularly etched for the aperture width W less than "a".
Whereas the piezoelectric material B can be substantially
perpendicularly etched for the aperture width W less than "b" ("b"
is larger than "a"). This material A can be PZT, and the width "a"
can be 2 to 3 .mu.m, and the width "b" can be about 20 .mu.m. As
explained above, the aperture width W should preferably be not more
than 20 .mu.m to perform the perpendicular etching.
[0109] (4) A step of removing redundant segments of piezoelectric
block:
[0110] As shown in FIG. 1K, spaces 27 are formed in the
piezoelectric block 25 by removing the redundant segments 62 of the
piezoelectric block 25. More specifically, the silicon layer 36 is
selectively etched away with XeF.sub.2 gas to thereby remove the
segments 62 of the piezoelectric block 25 disposed on the silicon
layer 36. In this case, the etching gas can enter from the surface
of the piezoelectric block 25 to reach the silicon layer 36 through
the holes or grooves 22 to thereby etch and remove the silicon
layer 36. As described above, XeF.sub.2 shows a much slower etching
rate for the materials other than Si as compared with the etching
rate for Si, so that the silicon layer 36 can be exclusively etched
away with excellent selectivity.
[0111] (5) A step of filling an organic material:
[0112] As shown in FIG. 1L, the spaces 27 are filled with an
organic material 2. The organic material 2 can be filled in such a
way that the sample shown in FIG. 1K is placed inside a closed
vessel, and, while evacuating the vessel, the organic material 2 is
introduced into the vessel through an another inlet port attached
to the vessel, thereby filling the spaces 27 with the organic
material 2. The organic material 2 thus introduced into the vacant
portions 27 is subsequently heated to cure.
[0113] (6) Steps of polishing, providing electrodes and providing
piezoelectricity:
[0114] The piezoelectric block 25 filled with the organic material
2 is abraded and polished from the surface thereof to an extent
indicated by a broken line A as shown in FIG. 1L to expose both of
the piezoelectric column 61 and the organic material 2. The
resultant composite substance consisting of the piezoelectric
column 61 and the organic material 2 can have a thickness of about
65 .mu.m.
[0115] Then, as shown in FIG. 1M, electrodes 39 are deposited on
the top and lower surfaces of the sample where the piezoelectric
column 61 and the organic material 2 are both exposed. Thereafter,
a DC voltage is applied between the electrodes 39 to execute the
poling treatment of the piezoelectric substance 25 to provide the
substance 25 with piezoelectricity. Finally, the shaping work of
the sample is performed to accomplish the composite piezoelectric
transducer.
[0116] Although the poling treatment is performed as the final
process in this embodiment, this treatment may be performed when
manufacturing the piezoelectric block. Care should be taken in this
case such that in the subsequent steps after the poling treatment,
the piezoelectric substance should not be heated up to the vicinity
of Curie point thereof, thereby preventing the piezoelectricity
thereof from being deteriorated.
[0117] FIG. 4 shows one example of the composite piezoelectric
transducer prepared as described above, wherein a perspective view
schematically illustrating the composite piezoelectric transducer
of 1-3 structure according to this first embodiment is shown. This
1-3 structure means a structure wherein the piezoelectric
substances are extending one-dimensionally in a longitudinal
direction, while the resin elements are extending
three-dimensionally. Namely, the composite piezoelectric substance
consisting of a piezoelectric substance and a resin can be
classified by the number of the dimension (direction) in which each
component is extending, i.e. the composite piezoelectric substance
can be represented by: (the number of dimension in which the
piezoelectric substances are extending)-(the number of dimension in
which the resin components are extending).
[0118] The composite piezoelectric transducer 70 shown in FIG. 4
comprises a plurality of piezoelectric columns 1 disposed parallel
with each other, an organic material 2 filled in the spaces between
piezoelectric columns 1, and electrodes (not shown) formed on the
top and lower surfaces, including the both end faces of each
piezoelectric column 1.
[0119] A plurality of piezoelectric columns 1 are spaced apart with
their longitudinal axes in parallel. Each column 1 has a first and
second end faces, which are substantially perpendicular to the
longitudinal axis thereof. The organic material 2 is filled in the
spaces between the piezoelectric columns 1 so as to expose the
first and second end faces. The electrodes have a first electrode
which is in contact with the organic material 2 and the first end
face, and a second electrode which is in contact with the organic
material 2 and the second end face.
[0120] Since the sidewall of the piezoelectric column 1 is worked
using a plasma dry etching method as described above, the sidewall
can be kept clean without cut chips or oily matters adhered, and
the surface thereof is modified. Thus, the adhesion strength can be
enhanced between the piezoelectric column 1 and the resin. The
enhanced adhesion strength can improve the bending strength of the
resultant composite piezoelectric transducer, and can provide a
composite piezoelectric transducer having a complicated
configuration such as a concaved shape. Since the mechanical
strength of the composite piezoelectric transducer can be improved,
it is possible to improve the durability of the final ultrasonic
probe.
[0121] FIG. 5 shows a cross-sectional view schematically
illustrating one example of a distal end portion of an ultrasonic
probe manufactured with the composite piezoelectric transducer 70
shown in FIG. 4.
[0122] The ultrasonic probe shown in FIG. 5 is constructed as
follows. An insulating pipe 6 is adhered to the inner surface of a
conductive housing 5, and a backing material 7 is filled in the
insulating pipe 6. The composite piezoelectric transducer 70 having
a curvature is fixed to the front face of the backing material 7.
An acoustic matching layer 9 is attached to the composite
piezoelectric transducer 70 on the front face thereof which is
designed to be an acoustic radiating face. Further, the front face
of the composite piezoelectric transducer 70 for the acoustic
radiating side is connected, through a conductive resin or a
combination of a solder and a lead wire 10, with the housing 5. The
back face (designed to be a signal side) of the composite
piezoelectric transducer 70 is connected, through a low temperature
solder 13, with a signal wire of external lead wire 11. The GND
(ground) wire 14 of the lead wire 11 is connected, through a low
temperature solder 13, with the housing 5.
[0123] The ultrasonic probe in FIG. 5 has the composite
piezoelectric transducer 70 which is manufactured by making use of
a plurality of piezoelectric columns 1 each having a cleaned
sidewall which is designed to be contacted with a resin as shown in
FIG. 4. As a result, the composite piezoelectric transducer 70 has
a high adhesivity with the resin 2, and a high durability and a
high resolution due to a high aspect ratio.
[0124] In the ultrasonic probe shown in FIG. 5, a low temperature
solder 13 is employed for the electrical connection between the
composite piezoelectric transducer 70 and the signal line 12, and
between the housing 5 and the GND wire 14, although it is possible
to employ a conductive resin, a sputtering method or a vapor
deposition method for these electrical connections.
[0125] Although a plasma etching is employed in the etching step in
the present embodiment, the etching may be performed using other
kinds of etching other than the plasma etching. For example, an
etching using a femto-second laser, or an etching using the
bombardment of ionized cluster may be employed.
[0126] Next, a second embodiment according to this invention will
be explained with reference to FIGS. 6A to 6D, 7, 8A and 8B. The
same members or elements as those in the first embodiment will be
identified by the same reference numerals, and, only different
features from the first embodiment will be mainly explained.
[0127] The steps prior to the plasma etching in the manufacturing
method according to this embodiment are the same as those in the
first embodiment shown in FIGS. 1A to 1H. Namely, a piezoelectric
block is prepared, and a Cr/Au electrode as an underlying layer is
formed on one surface of the piezoelectric block, after which a
resist is coated and subjected to the steps of patterning
exposure/development. Then, a nickel layer is formed by means of
plating, and the resist layer is removed to form a nickel mask.
Thereafter, a sacrificial layer 35 and an etch-stop layer 36 are
formed on the other surface of the piezoelectric block. Finally, a
supporting substrate 37 is bonded to the other surface, thereby
making it ready for the plasma etching step.
[0128] As a material for the piezoelectric block 25 employed in
this embodiment, a potassium niobate (KNbO.sub.3) plate which has
been cut out at an angle of 44 degrees relative to the polarization
axis is employed. Chromium (Cr) and then gold (Au) are successively
deposited by means of sputtering on one surface of the
piezoelectric block 25 to thereby form a thin film. A film of
nickel is then deposited on the thin film by means of plating while
passing a pulse current to the plating bath. Since nickel is
pulse-plated in this manner, it is possible to enhance the etching
selectivity during the plasma etching using a nickel mask. Further,
since the surface of the nickel mask can be made uniform, it is
possible to prevent the piezoelectric column from being tapered
during the etching.
[0129] The sacrificial layer 35 is formed by making use of a
resist. The etch-stop layer 36 is formed in such a manner that
after the deposition of an Au/Cr thin film, a nickel layer (3 .mu.m
in thickness) is formed, as the etch-stop layer 36, in a nickel
sulfamate bath.
[0130] As shown in FIGS. 6A and 6B, a plurality of piezoelectric
columns 61 and 62 are formed by means of plasma etching. As for the
reactive gas for the etching, a mixed gas comprising a Cl gas and a
CF.sub.4 gas can be employed. The etching is continued until it
reaches the resist layer 35 that has been formed in advance.
[0131] As shown in FIG. 6C, the sample is immersed in acetone or a
releasing solution exclusively adapted for the resist to thereby
remove the resist layer 35 and redundant segments of the
piezoelectric columns 62, thus obtaining a piezoelectric
structure.
[0132] As shown in FIG. 6D, the sample is cleaned by making use of
an oxygen plasma, and then, epoxy resin 2 having a Shore hardness
A90 or so after a curing thereof at room temperature is filled in a
space between neighboring piezoelectric columns 61. Then, both
surfaces of the sample are abraded so as to expose the resin 2 and
the group of piezoelectric columns 61. Then, the resultant surface
exposing the resin 2 and the piezoelectric columns 61 is cleaned by
means of an UV treatment or an oxygen plasma, and then, electrodes
39 are formed on the cleaned surface.
[0133] FIG. 7 shows a perspective view schematically illustrating
one example of the composite piezoelectric transducer of a 1-3
structure (the piezoelectric substances are one-dimensionally or
longitudinally extending, while the resin elements are
three-dimensionally extending) which has been prepared as described
above.
[0134] As shown in FIG. 7, a plurality of piezoelectric columns 61
are spaced apart with their longitudinal axes in parallel. Each
piezoelectric column 61 has a first and second end face, which are
substantially perpendicular to the longitudinal axis thereof. The
organic material 2 is filled in the spaces between the
piezoelectric columns 61 so as to expose the first and second end
faces. The piezoelectric columns 61 are varied in diameter, and
arranged only in the spaces between the upper and lower electrodes
39 (the spaces can be referred to "elements": portions to be driven
by the upper and lower electrodes). Since the diameters of the
piezoelectric columns 61 are different, these piezoelectric columns
61 can have different resonance frequencies, even when they have
the same lengths. As a result, the composite piezoelectric
transducer can have an enlarged frequency band to improve the image
quality in an image observation apparatus in which the transducer
is employed.
[0135] According to this embodiment, the piezoelectric columns 61
having different diameters are disposed below the electrodes 39 in
each element, and in the regions between elements where no
electrodes 39 are arranged, only the resin 2 is provided or no
piezoelectric columns 61 is arranged. Such an arrangement of the
piezoelectric columns 61 allows bending of the ultrasonic
transducer around the soft resin 2 portion between the elements, so
that a curved ultrasonic transducer can be manufactured without
cutting at the portions between the elements.
[0136] Further, since the piezoelectric columns 61 are manufactured
by means of etching, the columns 61 of desired configurations can
be arranged in desired locations, which is impossible in the case
where cutting such as dicing is employed. It is also possible
according to this invention to manufacture narrow piezoelectric
columns 61 of a 10 .mu.m in diameter by means of etching.
[0137] According the embodiment shown in FIG. 7, each of the
piezoelectric columns 61 is formed into a cylindrical
configuration. However, the piezoelectric columns 61 may be
hexagonal or any other polygonal configuration in cross-section as
shown in FIG. 8A. FIGS. 8A and 8B show respectively a plan view
representing one example of piezoelectric substance bearing thereon
a nickel mask 24. The nickel mask 24 is partitioned, by the
intervention of the aperture portion 26, into a mask portion 24a
corresponding to the piezoelectric column 61 to be left remained as
shown in FIG. 6B and a mask portion 24b corresponding to the
piezoelectric substance portion 62 to be removed. As described
above, the mask portion 24a may be formed to have a hexagonal or
any other polygonal configuration in cross-section as shown in FIG.
8A. Further, the piezoelectric columns 61 may be arranged
irregularly. For the purpose of reducing the transverse vibrations
that may cause a noise, it is preferable to make the distance
between the neighboring piezoelectric columns 61 non-uniform.
[0138] In order to facilitate the removal of the redundant segments
(the piezoelectric substance portion 62) of the piezoelectric
block, a slit (aperture portion) 26 of a width of several
micrometers partitioning the piezoelectric portion 62 into a
plurality of sub-portions may be formed in advance in the mask
portion 24a. Such a slit 26 is effective in improving the
yield.
[0139] Since the fine working is performed by means of a dry
etching, it is now possible to easily work a single crystal
piezoelectric substance (it is difficult to apply a mechanical
working such as dicing to this single crystal piezoelectric
substance, and it is impossible to apply a lost mold process
utilizing LIGA, etc. to this single crystal piezoelectric
substance).
[0140] Further, since a piezoelectric structure prepared through
the etching of a single crystal piezoelectric substance is
utilized, it is now possible to manufacture a composite
piezoelectric transducer which is very excellent in
piezoelectricity. Therefore, the ultrasonic probe prepared by
making use of this composite piezoelectric transducer would exhibit
an improved S/N ratio, thereby making it possible to achieve a high
resolution.
[0141] Although an array type transducer where the electrode is
partitioned has been explained in this embodiment, the transducer
is not confined to such an array type one. Namely, this invention
is also applicable to a transducer provided with a pair of
electrodes on both surfaces thereof, or to an annular array where
the electrode is partitioned annularly.
[0142] Next, a third embodiment according to this invention will be
explained with reference to FIGS. 9A, 9B, 10A and 10B. The same
members or elements as those in the first embodiment will be
identified by the same reference numerals, and hence, only the
different features from the first embodiment will be mainly
explained.
[0143] The manufacturing method according to this embodiment is
fundamentally the same as that in the first embodiment illustrated
in the steps shown in FIGS. 1A to 1M except that a piezoelectric
ceramic Pb(Zr, Ti)O.sub.3 plate is employed for the piezoelectric
block 25 and a plural of grooves are formed on the ceramic plate to
prepare a composite piezoelectric substance of 2-2 structure (the
piezoelectric bodies and the resin elements both are
two-dimensionally extending).
[0144] Namely, the piezoelectric block 25 is prepared, and an
underlying layer for forming electrodes is formed on one surface of
the piezoelectric block, after which a resist is coated and
subjected to the steps of patterning exposure and development.
Then, a nickel layer is deposited thereon by means of plating, and
the resist layer is removed to form a nickel mask. Thereafter, a
sacrificial layer 35 and an etch-stop layer 36 are formed on the
other surface of the piezoelectric block, after which a supporting
substrate 37 is bonded to the other surface of the piezoelectric
block 25, thereby making it ready for the plasma etching step.
[0145] Then, the piezoelectric block 25 is subjected to an etching
process employing a parallel plate RIE (reactive ion etching)
method to thereby manufacture a plurality of piezoelectric columns
61 and 62 as shown in FIG. 1J. Subsequently, the resist layer 35 is
etched away to thereby remove the redundant segments (piezoelectric
substances 62) of the piezoelectric block 25 are removed.
Thereafter, the grooves 27 are filled with the resin 2 in the same
manner as explained in the first embodiment, which is followed by
the step of abrasion to a desired thickness and the steps of
providing electrodes and executing the poling, as shown in FIG.
1M.
[0146] FIG. 9A shows a perspective view schematically illustrating
one example of the composite piezoelectric transducer of 2-2
structure which has been prepared as described above. The
electrodes are omitted in FIG. 9A.
[0147] After providing a backing material or an acoustic matching
layer, the composite piezoelectric transducer of 2-2 structure
shown in FIG. 9A is cut along the line B perpendicular to the
longitudinal direction to form an array as shown in FIG. 9B,
thereby obtaining almost the same structure as that of the
composite piezoelectric transducer of 1-3 structure shown in FIG.
7.
[0148] As shown in FIG. 10A, as long as at least part of the spaces
27 such as the groove and hole is filled with the resin 27, it is
possible to obtain the properties demanded of the composite
piezoelectric transducer. This partial filling of the resin 2 in
the spaces 27 can be performed in such a way that the surface of a
separately prepared resin 2 is brought into contact with the
opening of the spaces 27 where the resin is not filled as shown in
FIG. 1K, thereby filling the resin 2 only in the vicinity of the
opening of the spaces 27. Thereafter, the sample is abraded and,
electrodes are provided thereto to obtain a composite piezoelectric
transducer in which the resin 2 is partially filled as shown in
FIG. 10A. The air left within the spaces 27 as shown in FIG. 10A
can minimize the cross-talk between the piezoelectric columns 61
and can lower the acoustic impedance to enhance the permeability of
sound into human body, as described above. Therefore, the image
quality can be improved in the ultrasonic diagnostic apparatus
wherein this composite piezoelectric transducer is employed.
[0149] As described above, the etching may be performed so as to
provide a tapered sidewall to the piezoelectric substance by
changing the etching conditions. Thus, it is possible to provide a
probe of enhanced S/N ratio and hence, enhanced sensitivity. FIG.
10B shows one example of the composite piezoelectric transducer
comprising arrayed piezoelectric columns 61 which are tapered and
varied in size. The provision of tapering to the sidewall of
piezoelectric substance is also effective to the aforementioned
first and second embodiments.
[0150] Although three kinds of embodiment are explained above, the
constituent elements of these embodiments can be variously
modified. Typical examples of such modifications will be
illustrated as follows.
[0151] The piezoelectric columns 61 all having the same height are
manufactured at first, and after the organic material 2 is filled
therein, one or both surfaces of the piezoelectric columns 61 are
abraded or polished so as to obtain a composite piezoelectric
transducer having a convex or concave surface or surfaces.
[0152] A plurality of piezoelectric substances 1 may be
manufactured in such a manner that at least one of the
piezoelectric substances 1 differs in diameter from that of the
other piezoelectric substances 1.
[0153] A plurality of piezoelectric bodies 1 may be manufactured in
such a manner that at lease one of the piezoelectric substances 1
differs in length from that of the other piezoelectric substances
1. Thus, it would become possible to manufacture a composite
piezoelectric transducer wherein only one surface thereof is formed
into a convex surface or a concave surface. Alternatively, it would
become possible to manufacture a composite piezoelectric transducer
wherein both surfaces thereof are formed into a convex surface or a
concave surface.
[0154] It is also possible to combine the aforementioned two
methods; the method of forming the piezoelectric substances 1 in
the manner that at least one of the piezoelectric substances 1
differs in diameter from that of the other piezoelectric substances
1, and the method of forming the piezoelectric substances 1 in the
manner that at least one of the piezoelectric substances 1 differs
in length from that of the other piezoelectric substances 1.
[0155] Of course it is also possible to further change or modify
each constituent feature of the aforementioned various embodiments
of this invention.
[0156] Although this invention has been explained based on the
aforementioned embodiments, this invention also includes the
following features.
[0157] A method of working a block-shaped piezoelectric block by
means of plasma etching which is featured in that the etching work
of the piezoelectric block is performed by making use of an
apparatus comprising a high-frequency power source which is capable
of generating and maintaining plasma, and a low-frequency power
source which is capable of applying a frequency of not more than 1
MHz to the stage.
[0158] A method of manufacturing a composite piezoelectric
substance which comprises the steps of:
[0159] forming a mask of desired configuration at a desired portion
of a piezoelectric substance formed of potassium niobate single
crystal;
[0160] etching the piezoelectric substance by means of plasma;
[0161] removing redundant segments of the piezoelectric substance
which have been separated in the etching step;
[0162] filling at least partially the spaces formed after the
removal of the redundant segments of the piezoelectric substance
with an organic material; and
[0163] providing electrodes to the composite piezoelectric
substance.
[0164] As explained above, it is possible, according to this
invention, to manufacture a fine and highly densified composite
piezoelectric transducer. Further, since the piezoelectric
substance to be removed by means of etching can be reduced, any
damage can be minimized for an etching apparatus, a facility for
removing exhaust and environments. Further, this invention is also
applicable to a piezoelectric single crystal. Additionally, it is
now possible to provide a fine and highly densified composite
piezoelectric transducer which is excellent in adhesivity between
the resin and the piezoelectric substance. As a result, it is now
possible to improve the durability of the composite piezoelectric
transducer, and when this composite piezoelectric transducer is
employed in an ultrasonic probe, it becomes possible to provide an
image which is excellent in sensitivity and resolution.
[0165] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *